Material Selection Considerations for Lightweight Automotive Bodies

Material Selection Considerations for Lightweight Automotive Bodies

Herman Tang (Eastern Michigan University, USA)
DOI: 10.4018/978-1-7998-4939-1.ch008
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Abstract

Lightweight is a trend of new vehicle development, driven by government regulations, environmental concerns, and customer needs. A major effort in the automotive industry is on light weighting vehicle bodies. This chapter reviews the various materials, their characteristics, weight reduction potentials, and costs for light weighting vehicle bodies. The chapter also exams the joining technologies on their principles, applications for the lightweight materials, and influencing factors for choosing a joining process. Furthermore, this chapter discusses the development trends of material selection and joining technology applications.
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Introduction

Demands of Weight Reduction

The better fuel economy of passenger vehicles is required by the US government and customers. The Corporate Average Fuel Economy (CAFE) standards are regulations in the United States. In 2011, CAFE required the improved fuel economy to 54.5 mpg for the cars and light-duty trucks in US by 2025 model year. The target 54.5 mpg is still under discussion and debates as EPA and NHTSA proposed new Safer Affordable Fuel-Efficient (SAFE) Vehicle Rules to replace the original CAFE for mode year 2021-2026 (DOT 2018). The CAFE/SAFE standards have been a main driving force for the automakers to seek various ways to improve the fuel consumption of new vehicles.

Light-weighting is an important path and approach to achieve a better fuel economy for automakers to meet these standards. It is reported that reducing a vehicle’s mass by 10% can improve fuel economy by 6-8% (DOE, 2015). In addition, for new electric cars, their weight is extremely important to the battery power demand. A study showed that a 10% reduction in vehicle weight permits nearly a 10% smaller in battery size (Smock, 2010). Another study showed that 100kg weight reduction from a 1593-kg electric vehicle might improve the total cost of ownership by €500 over a 4-year holding period (Redelbach, 2012).

A midsize passenger car weights about 3400 lb (1542 kg). Many industrial experts consider a 450 lb (204 kg), or about 13%, reduction of vehicle weight is necessary by 2025. To reduce the total weight of a vehicle, its body structure is often considered a main target for a 30% reduction. The other segments of a passage vehicle have similar targets. For example, powertrain units may be expected to be reduced by 25%. Other units, such as chassis, interior, closures, and glazing, may be targeted for 20, 14, 8, and 3%, respectively.

Environmental Concern

The emission of CO2, sometimes called Greenhouse gas (GHG), is a main environmental concern. The transportation is a large contributor to GHG emissions. Figure 1 shows a clear correlation between the fuel economy and CO2 emission in the last four decades (EPA, 2019). Therefore, the fuel economy improvement directly benefits the GHG reduction.

Figure 1.

EPA report on vehicle fuel economy and CO2 emission

978-1-7998-4939-1.ch008.f01

The CO2 emission from vehicle fuel combustion can be calculated by

978-1-7998-4939-1.ch008.m01

Where, CO2 emission and gas combusted are in weight, a constant 0.87 is due to 87% carbon and 13% hydrogen in gasoline by weight, the 44 is molecular weight of CO2, and the 12 is molecular weight of carbon. One gallon of gasoline weights about 6.3 lb. Therefore, one-gallon gasoline can produce approximately 20 lb (9.07 kg) CO2 when burned (DOE, 2016).

Another factor, the CO2 generated from the material production, should be considered as well. Lifecycle assessment (LCA) is a good tool to evaluate the CO2 emission for material selection. If an LCA study includes the material production, the analysis results can be different from those only focusing on vehicle fuel combustion. According to the World Steel Association, steel production generates much less GHG than other materials for vehicle bodies, refer to Table 1 (ArcelorMittal, 2014). From this perspective, taking the vehicle mass reduction rate (say up to 50% if using aluminum) into count and the CO2 from material production, steels can be competitive for the purpose of CO2 emission reduction.

Table 1.
Greenhouse gas generated from material production
MaterialGHG (kg CO2e/kg)
Steels2.0–2.5
Aluminum Alloys11.2–12.6
Magnesium Alloys18–45
Carbon Fiber21–23

Key Terms in this Chapter

Corporate Average Fuel Economy (CAFE): CAFE standards are the regulations of the National Highway Traffic Safety Administration (NHTSA), the US Department of Transportation. CAFE standards regulate how far our vehicles must travel on a gallon of fuel. In 2019, NHTSA and EPA establish the One National Program for fuel economy regulation Safer Affordable Fuel-Efficient (SAFE).

Vehicle Body Assembly Processes: Body assembly processes are primarily joining processes. In the processes, the parts are positioned properly first, and then joined using various types of joining processes. For a traditional vehicle body made of steel sheet metal parts, the processes are commonly welding and adhesive bonding. Adopting new materials and body design can significantly change the processes.

Body in White (BIW): BIW is a body frame structure of a vehicle without paint. Traditionally, a body structure is built with steel sheet metal parts. The “white” refers to the metallic appearance or color of steel surface even the color is not actually white. A BIW often does not include closure panels, such as doors, fenders, hood, and deck-lid.

Vehicle Mass Production: For vehicle assembly, mass production may be called high-volume production as well. Two main characteristics of mass production are standardized products and processes, and a high level of automation. For vehicle BIW assembly, the automation level can be higher than 90% for mass production. The annual production output of a vehicle assembly plant, running two shifts a day, five days a week, is normally 150,000 – 300,000 units a year.

Greenhouse Gas (GHG): GHG mainly refers to CO 2 and includes other gases. CO 2 contributes to the greenhouse effect. Once emitted, GHG rises into the air, stays in the atmosphere for a period, and traps energy from the sun as heat. Thus, excessive GHG causes earth to warm up.

Ultra-High-Strength Steels (UHSS): UHSS are the steels that have very high strength, such as over 700 or 780 MPa. They are also called Advanced High-Strength Steel (AHSS).

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